Process for producing steels according to the oxygen top-blowing method

ABSTRACT

The invention relates to a process for producing steels, in particular soft, rimming, chromium containing deep drawing steels from chromium containing pig iron according to the oxygen topblowing method. Before or at the onset of blowing 5 to 50 kg of solid chromium-oxide free slag with an analysis of Fe? 10-20%? SiO28-15% Mn4-10% CaO45-55% MgO1- 5% Al2O31- 5% P2O51- 3% and a grain size of 3 to 20 mm, are added to the pig iron per metric ton of metallic input, whereupon within the period of 5 to 20 percent of the total blowing period an amount of CaO is added into the converter as is necessary for reducing to slag any accompanying elements of the pig iron and for adjusting a desired degree of slag basicity. With this process the formation of liquid reactive slag is achieved more rapidly and the time for servicing the blowing lance is reduced. Yield losses are prevented and the life of the refractory lining of the converter is prolonged.

United States Patent [1 1 I,

Rinesch et a].

[ 51 Apr. 9, 1974 PROCESS FOR PRODUCING STEELS ACCORDING TO THE OXYGENTOP-BLOWING METHOD [73] Assignee: Vereinigte Osterreichische EisenundStahlwerke Aktiengesellschaft, Linz, Austria [22] Filed: Feb. 17, 1972[21] Appl. No.: 227,304

[30] Foreign Application Priority Data Mar. 1, 1971 Austria 1713/71 [52]U.S. Cl 75/52, 75/55, 75/60 [51] Int. Cl C2lc 5/32, C2lc 5/36 [58] Fieldof Search 75/52-58, 59, 75/60 [56] References Cited UNITED STATESPATENTS 2,440,564 4/1948 Allard 75/52 3,376,130 4/1968 Kootz 75/52Primary Eraminer-Charles N. Lovell Assistant Examiner-Peter D. RosenbergAttorney, Agent, or Firm-Brumbaugh, Graves, Donohue & Raymond 57 7ABSTRACT The invention relates to a process for producing steels, inparticular soft, rimming. chromium containing deep drawing steels fromchromium containing pig iron according to the oxygen top-blowing method.Before or at the onset of blowing 5 to 50 kg of solid chromiumsidei eseey t a analysis of and a grain size of 3 to 20 mm, are added to the pigiron per metric ton of metallic input, whereupon within the period of 5to 20 percent of the total blowing period an amount of CaO is added intothe converter as is necessary for reducing to slag any accompanyingelements of the pig iron and for adjusting a desired degree of slagbasicity. With this process the formation of liquid reactive slag isachieved more rapidly and the time for servicing the blowing lance isreduced. Yield losses are prevented and the life of the refractorylining of the converter is prolonged.

4 Claims, No Drawings PROCESS FOR PRODUCING STEELS ACCORDING TO THEOXYGEN TOP-BLOWING METHOD The invention relates to a process forproducing steels according to the oxygen top-blowing method, inparticular for producing soft, rimming, chromium containing deep drawingsteels from chromium containing pig iron, wherein basic slag formersand, if desired, fluxes are added for the formation of a fluid, reactiveslag.

In the production of steel according to the oxygen top-blowing methoddifiiculties arise when a pig iron is refined that contains accompanyingelements which are slagged to oxides that melt only with difficulty,whereby the formation of a fluid, reactive slag is delayed. Such a pigiron is for example a chromium containing steel pig iron containingabout 4 percent C, 0.5 to 0.8 percent Si, 1.4 percent Mn, up to 0.2percent P, up to 0.04 percent S and 0.9 to 1.2 percent Cr. It is blownwith 0.2 to 0.4 percent chromium for the production of soft, rimmingdeep drawing steels that are resistant to ageing; see Rinesch R.: DieVerhuttung chromhaltiger Eisenerze bis zur Herstellung kaltgewalzterunberuhigter, alterungsbestandiger Tiefziehbleche mit 0.2 0.4 percentChrom, Bergund Huttenmannische Monatshefte l 12 (1967), pages 402/412.ln blowing chromium containing pig iron so far a reduced life of theconverter lining has been taken for granted for it is necessary to addfluxes to the slag when the process starts; the slag becomes viscuousand crumbly by the forming of Cr O These fluxes then lead at a highertemperature to the formation of too fluid a slag, which is highlyaggressive and strongly attacks the refractory lining. A further problemwhich also is connected with the retarded formation of a liquid slag isthe fact that a skull is formed at the blowing lance. So long as theslag is still dry," i.e., crumbly, the converter sprays, whereby irondroplets are flung upwardly and out of the converter. Thereby at thelower part of the blowing lance deposits of metal and slag are formedwhich have to be removed after each melt, a task that is highlydifficult and necessitates a production interruption. Furthermore theseskull formations may lead to a damage of the blowing lance. Similardifficulties occur when a pig iron poor in silicium and/or manganese,e.g., a pig iron comprising about 4 percent C, less than 0.3 percent Si,less than 0.6 percent Mn, up to 0.2 percent P and up to 0.04 percent S,is blown. In both pig iron sorts, a liquid slag that ends spraying isformed as a rule only 8 minutes after onset of blowing, i.e., afterabout 60 percent of the total blowing time, which amounts to 14 minutes.

Numerous attempts and proposals have been made to accelerate theformation of a fluid slag in the oxygen top-blowing method. Thus, it ispossible e.g., to increase the distance between the bath surface and theblowing lance and to blow soften however, then the CO formed duringrefining is partially burnt in the con verter so that the refractorylining in the upper part of the converter is worn to a greater degree.According to another proposal fine granular fluxes, such as soft burntlime, are continuously blown onto the bath together with the blowingstream as this is described e.g., in the British patent specificationNo. 883,958 and US. Pat. No. 3,004,847. However, for supplying the solidmaterials an expensive apparatus is necessary which is also liable todisturbances. Furthermore such solid supply plants necessitate partly anextremely fine granular material the preparation of which is expensive.The supply of dust-like lime for a rapid slag formation is thereforeonly justifiable when pig iron sorts containing a high amount ofphosphorus are refined, because this solves above all metallurgicaldifficulties relative to dephosphorization and there is no problem ofskull formation at the blowing lance. From the British patentspecification No. 883,958 and US. Pat. No. 3,004,847 it is furthermoreknown that in refining Thomas pig iron, which takes place in two phases,the slag containing a high amount of P 0 being drawn off in between, thefinal slag is retained in the converter and used for the next charge.This liquid end slag as a rule contains a high amount of CaO so that itis used for the next charge also in order to save lime. Working withretaining the final slag or part thereof leads to difficulties when theliquid pig iron of the following charge is filled into the converterbecause strong, even explosion-like reactions may occur between thecarbon of the pig iron and the iron oxide of the slag. A furtherdisadvantage lies in that the weight and the composition of the finalslag does not remain constant so that disturbances may arise in thecourse of the process of the following melt and faulty analyses may beobtained. Also the temperature of the end slag and its viscosity may bevery different at the beginning of blowing of the subsequent m'elt sothat great delays may occur with regard to the period when a reactiveslag starts to be formed. Apart from these operational difficulties,this process, in which the final slag is used again for the followingmelt, cannot be used in refining pig iron sorts containing about 1percent chromium, because they are refined in a one-step process so thatthe end slag contains the overall amount of the chromium reduced to slagas Cr O and thus cannot be used again. For overcoming the difficultiesand for accelerating the slag formation in oxygen top blowing convertersthe German provisional publication No. 1,608,310 suggests the use ofsynthetic slag formers instead of powderized lime or pieces of lime orlimestone; these slag formers are to be produced from a mixture of acalcium oxide with powderized, partly or completely sintered iron ore.The production of synthetic slags necessitates certain raw materials andis expensive so that its use has not been justified on a largeindustrial scale. According to a further proposal synthetic slagproduced from red mud obtained from aluminum industry, from dolomite,lime and bauxite, is used in an oxygen top blowing converter in order toincrease the life of the refractory lining by providing for a rapidformation of a fluid slag.

Finally, in this connection a proposal contained in the German printedapplication No. 1,783,013 should be mentioned, according to which, inthe production of steel with carbon contents of 0.3 to 1.0 percent ascatch charges according to the oxygen top-blowing method, for obtaininglow P-contents part of the lime demand is added in the form of alime-iron oxide mixture, which is also to be produced synthetically,within a period starting before onset of blowing and lasting until halfof the blowing time; pellets of lime and red mud or of lime and blowingdust mud are to be produced and used; this, however, is likewiseexpensive.

The invention is aimed at creating an inexpensive process that mayeasily be carried out, with which the formation of skulls at the blowinglance and losses of yields by spraying of the metal at the onset ofblowing are prevented and with which also the life of the refractorylining of the converter is to be improved.

Thus, in a process of the kind defined in the introduction, theinvention resides in that to the pig iron before or at the onset ofblowing per metric ton of metallic input 5 to 50 kg, preferably 20 to 30kg, of solid chromium oxide free slag having an analysis of Fe -20% si028-! 5% Mn 4- i 0% CaO 45-5 5% MgO 1-5 7. ALO, l-5% 150 14% and a grainsize of 3 to mm, preferably 3 to 8 mm, are added, whereupon within aperiod of 5 to 20 percent of the total blowing period, calculated fromthe onset of blowing, an amount of CaO in the form of burnt lime isadded into the converter as is necessary for reducing to slag anyaccompanying elements of the pigiron and for adjusting a desired degreeof slag basicity.

Preferably a slag is used whose pyrometric cone fall point (Seger) liesbetween l,300 and l,370C and whose flow point lies below l,380C.

Individual analyses of slags used in operational charges are listedbelow; their pyrometric cone fall point (Seger) lies between l,340 andl,350C and their flow point lies at 1,360C.

Such slags are obtained when chromium free steel pig iron sorts ofcustomary composition are refined; since these slags are inexpensive,only the costs for their pulverisation have to be calculated for theiruse according to the invention. Thus a mill product may be used that isalready present and need not be produced synthetically; in the scope ofthe process according to the invention it is its main task to form aslag protective layer preventing spraying at the onset of blowing sothat the blowing lance is protected. Since in such steel work slags theCaO is for its greatest part bound to the remaining oxides, and the freelime portion participating in the metallurgical reactions is negligible,such a slag functions essentially only as a protective layer against thespraying of the converter before the metallurgical slag proper isformed. The solid chromium oxide free steel work slag may also be addedalready into the pig iron charge ladle so that it is preheated at thebeginning of blowing and is liquefied yet more quickly. Only after theonset of blowing, namely within the period of 5 to 20 percent of thetotal blowing time, calculated from the onset of blowing, the limenecessary for the metallurgical reaction (dephosphorising,desulphurisation), is added, if desired with the addition of fluxes.When calculating the lime amount, obviously the Ca0- amount contained inthe added solid slag is considered.

In blowing chromium containing pig iron sorts it has been possible bythis method to reduce the time for the formation of a liquid reactiveslag by at least 2 minutes; the slag formation thus may be ended withina period of about 40 percent of the overall blowing time. Whilecustomarily ore is added at the beginning of blowing to provide for amore rapid liquefaction of the slag this is no longer necessary andresults in an increase of the scrap amount and thus in a reduction ofthe input costs. Also the addition of fluxes, which are alwaysunfavourable for the refractory lining, may be reduced when a solid slagis used. Finally the reduction of the time necessary for servicing,i.e., for removing skull formations at the lance, is expressed in aconsiderable increase in efficiency. Also the addition offerro-manganese into the casting ladle in the production of rimming deepdrawing steels with 0.2 to 0.4 percent chromium may be lowered, because,when solid slag is used for the formation of a protective layer for thepig iron bath, the

outthrow of metal and slag droplets is prevented to a large degree,which droplets at the beginning of the process have a high manganesecontent; the manganese content of the steel is therefore somewhat higherprior to tapping than with steels which are produced in customarymanner. In the following table operational results are listed whichillustrate the advantages of the process according to the invention.

without the with the addition of addition of solid slag solid slagnumber of mclts (30 mt tapping 243 267 weight) total servicing timeowing to skull 455 min 60 min formation at the blowing lance number ofmelts/day 42.8 48.8 yield pig iron good blocks 83.34 88.69 input per mtblock steel fluid pig iron 944.37 kg 892.02 kg scrap 206.47 kg 260.37 kgFe from ore 13.83 kg 0 kg lime 50.60 kg 43.37 kg fluorspar 4.48 kg 2.70kg solid slag (3-8 mm) 0 kg 28.36 kg ladle input per rnt block steelFerro-manganese 1.82 kg What we claim is:

l. A method for producing steel from pig-iron containing chromium by asingle continuous oxygen topblowing process carried out in a converter,comprising the steps of:

adding to said pig-iron, prior to blowing, 5 to 50 Kg of solidchromium-oxide free slag per metric ton of metallic input, said slagbeing selected from basic slag formers and fluxes and having an analysisof Fe 10-20% Ste, 8-] 5% Mn 4-1o% CaO 45-55% MgO l-5% Ago, 1-s% P 0, 14%

and a grain size of 3 to 20 mm,

blowing oxygen into the converter, and

simultaneously therewith adding within a period of 5 to percent of thetotal blowing time, calculated from the onset of blowing, an amount ofCaO in the form of burnt lime as is necessary for reducing to slag anyaccompanying elements of the pig-iron and for adjusting a desired degreeof slag basicity.

2. The process set forth in claim 1, wherein said slag is added in anamount of 20 to kg per metric ton of

2. The process set forth in claim 1, wherein said slag is added in anamount of 20 to 30 kg per metric ton of metallic input.
 3. The processset forth in claim 1, wherein the grain size of said slag amounts to 3to 8 mm.
 4. The process set forth in claim 1, wherein a slag is usedwhose pyrometric cone fall point lies between 1,300* and 1,370*C andwhose flow point lies below 1,380*C.